In conventional orthodontic treatment, an orthodontist or an assistant affixes an orthodontic appliance, such as orthodontic brackets, to the patient's teeth and engages an archwire into a slot of each bracket. The archwire applies corrective forces that coerce the teeth to move into orthodontically correct positions. Traditional ligatures, such as small elastomeric O-rings or fine metal wires, are employed to retain the archwire within each bracket slot. Due to difficulties encountered in applying an individual ligature to each bracket, self-ligating orthodontic brackets have been developed that eliminate the need for ligatures by relying on a movable latch or slide for captivating the archwire within the bracket slot.
Conventional orthodontic brackets are ordinarily formed from stainless steel, which is strong, nonabsorbent, weldable, and relatively easy to form and machine. Patients undergoing orthodontic treatment using metal orthodontic brackets, however, may be embarrassed by the visibility of metal, which is not cosmetically pleasing. To address the unsightliness of metal brackets, certain conventional orthodontic brackets incorporate a bracket body of a transparent or translucent non-metallic material, such as a clear or translucent polymer or a clear or translucent ceramic that assumes the color or shade of the underlying tooth.
Alternatives to orthodontic brackets include other clear or less visible appliances. The most common of these are the so-called “aligners” that are interchangeable by the patient during treatment. The clinician may prescribe a series of aligners, which are generally placed over but are not themselves adhesively secured or otherwise attached to the patient's teeth, to move one or more teeth from their original position to their aesthetically pleasing position. Typically, a series of aligners is required to fully treat the patient because the degree of movement produced by an aligner is limited. As such, when used in a series, each aligner in the series may be designed to fulfill a portion of the treatment process or move one or more teeth over a portion of the entire distance toward the desired position.
One such aligner is the Invisalign® system available from Align Technology, Inc. The Invisalign® system includes removable aligners that are to be worn by the patient. Generally, these aligners are clear or transparent polymer orthodontic devices that are removably positioned over the teeth of the maxilla and/or the teeth of the mandible. By this system, as treatment progresses, the patient removes one aligner having a specific prescription and replaces it with another aligner having a second, different prescription. Each aligner is responsible for moving the teeth toward their final predetermined or aesthetically correct position.
The Invisalign® aligners are fabricated by way of a physical and computer aided molding process. The method begins by forming an impression of the patent's dentition using a suitable impression material, such as polyvinylsiloxane (PVS). The impression is scanned by CT from which a computer creates a three-dimensional digital positive model of the patient's teeth and gingival. To create an appliance to reflect the next desired tooth configuration in the series of treatment stages, a new three-dimensional model must be created that reflects the patient's dentition in the desired configuration. This involves capturing the shapes of individual teeth from the three dimensional plaster model into a computer aided design system. In the computer aided design system, the teeth are separated by a computerized process, and then reset in the desired configuration. The resulting computerized model of the patient's dentition in a desired configuration is then used to print a physical model of that tooth configuration. Finally, clear plastic which will form the aligner, such as a polyurethane, is molded over the physical model of the tooth configuration. Subsequent physical steps trim the molded aligner to remove sharp edges or portions which might contact and irritate the gingiva. In addition, the aligner surface and edges are typically smoothed via a process such as tumbling.
In some cases, attachments facilitate coupling of the aligners to the teeth. In the Invisalign® process, the attachments are placed using a template aligner that is thermoformed over a mold that matches the initial positions of the patient's teeth. Thin material can be used for the template aligner so that the template aligner is not as stiff or retentive as normal aligners. A “well” or “pocket” is created in the template aligner for each attachment. These “wells” are loaded with attachment composite and the template is fully seated on the patient's arch. Each attachment is then cured to the manufacturer's instructions. When all the attachments in the template have been cured, the template is removed and any flashing is removed with a carbide or stone finishing bur.
As can be seen, the fabrication of aligners is a tedious process which compounds both cost and time of treatment for the patient. Since such an orthodontic treatment may require, for example, 25 intermediate reset molds to represent 25 stages of treatment progress, the cost and time required for the necessary steps of mold making, aligner formation, and trimming, may be prohibitively high. The cost is additive, as each new stage in treatment or each change in treatment requires the production of a new mold. Likewise, the cost of storing a series of molds for each patient throughout treatment may be formidable.
U.S. Pat. No. 5,975,893, which is assigned to the proprietor of the Invisalign® product, and incorporated by reference herein in its entirety, describes the process elaborated above, and further, at end of the invention summary at col. 7 lines 15-29, the '893 patent states: “methods for fabricating a dental appliance according to the present invention comprise providing a first digital data set representing a modified tooth arrangement for a patient. A second digital data set is then produced from the first digital data set, where the second data set represents a negative model of the modified tooth arrangement. The fabrication machine is then controlled based on the second digital data set to produce the dental appliance. The fabrication machine will usually rely on selectively hardening a non-hardened resin to produce the appliance. The appliance typically comprises a polymeric shell having a cavity shape to receive and resiliently reposition teeth from an initial tooth arrangement to the modified tooth arrangement”. The '893 patent provides no disclosure accompanying this statement, to indicate how to form the shape of such an appliance, or what material to use, and in commercial implementations, the owner of the '893 patent has always indirectly formed aligners from tooth models which are made by stereolithography as is described in the paragraphs preceding this one.
Treatment of malocclusion by aligners faces challenges other than the difficulty of manufacture. Specifically, aligners fastened with attachments may prove very difficult to install, as a result of the limited number of shapes that the attachment apertures on the aligner may take consistently with the Invisalign® manufacturing process; specifically, the attachment apertures must be formed by thermoforming over a stereolithographically-generated positive tooth model, which limits the type of apertures that may be made. Moreover, aligners may bind with the attachments and prove very difficult to remove.
Furthermore, in many aligner patients, the presence of the aligner within the patient's mouth causes a change in the points of occlusion between the mandible and maxilla, and in particular, causes the guidance of occlusion to move to the rear molars. This opens the patient's bite and typically intrudes the rear molars as a consequence of the unbalanced occlusion force on the rear molars. One result of this can be TMJ injury after the removal of the aligner because the force of the mandible is no longer resisted by the rear molars in the absence of the aligners.
For many patients aligners fabricated manually or by thermoforming on a positive model are uncomfortable and can irritate the patient's gingiva and/or tongue to such an extent that the soft tissue becomes inflamed and can potentially bleed. This discomfort is generally caused because the aligner is trimmed inaccurately to the patient's gingival margin. The inaccuracy in trimming is generally caused by the minimum size of the trimming tool particularly on the anterior lingual side where the aligner interferes with the tongue. Other anatomy such as the incisive papilla if not generally considered when trimming the aligner which can cause swelling or inflammation. In addition, the location where the aligner is trimmed can cause a sharp flange to be created at the base of the aligner near the gingival margin particularly on the lingual side.
It would be desirable to provide an alternative apparatus and methodology for realizing aligners configured to correspond to a series tooth configurations. Such apparatus and methods should be economical, reusable, reduce time consumption, reduce material waste, and, in particular, should reduce the need for fabricating multiple casts of teeth arrangements for stages in the orthodontic treatment. At least some of these objectives, as well as others, are met by the apparatus and methods of the invention described hereinafter.